1. Field of the Invention
The present invention relates to a laminated printed circuit board that has at least two layers and that are used for electronic devices such as information processing devices and communicating devices, in particular, to a laminated printed circuit board having a function for suppressing an undesired electromagnetic wave from radiating.
2. Description of the Related Art
In advanced information society, undesired electromagnetic waves radiated from electronic devices such as information processing devices and communicating devices disturb broadcasting and radio communication systems and cause devices to malfunction. To suppress undesired electromagnetic waves from radiating and prevent them from entering such devices, shields and filters are used.
As a method for suppressing an undesired electromagnetic wave radiated from an electronic device, an electromagnetic wave radiated from a printed circuit board as a radiation source is enclosed with a shield. Alternatively, a printed circuit board is suppressed from radiating an electromagnetic wave.
For example, in a printed circuit board disclosed as Japanese Utility Model Registration Laid-Open Publication No. 05-13095, as shown in FIG. 20, a ground layer of a printed circuit board 101 is electrically connected to a cage 107 through a leaf spring 102 and a metal guide rail 103 so that the voltage of the ground layer is the same as the voltage of the cage 107. In this structure, the ground layer 104 functions as a metal plate so as to prevent electromagnetic noise from leaking out.
In an IC card disclosed as Japanese Patent Laid-Open Publication No. 07-192105, as shown in FIG. 21, a printed circuit board 113 and a metal panel 114 oppositely disposed with a frame 112 is electrically connected with a ground terminal 115 secured to the metal panel 114. Thus, the metal panel 114 effectively functions as a shield against external electrostatic induction and electromagnetic induction. Consequently, an IC chip 113 can be prevented from malfunctioning and breaking against interference of an undesired electromagnetic wave and radiation of static electricity.
In a liquid crystal displaying device 120 disclosed as Japanese Patent Laid-Open Publication No. 06-82803, as shown in FIG. 22, a metal plate 122 is electrically connected to a ground line of a printed circuit board 121. Thus, a ground line voltage is stably supplied. In addition, a liquid crystal displaying screen 124 and a metal frame 123 function as shields for suppressing an undesired electromagnetic wave from radiating.
As a main cause of which a printed circuit board radiates an undesired electromagnetic wave, a voltage between a ground for supplying a reference voltage and a power supply layer for supplying a power supply voltage to an IC and so forth fluctuates. In particular, when a system composed of a power supply layer and a ground layer resonates, an electromagnetic wave in very high level is radiated. To suppress a radiation due to the fluctuation of a power supply voltage, several structures of printed circuit boards have been proposed.
In a printed circuit board disclosed as Japanese Patent Laid-Open Publication No. 06-224562, as shown in FIG. 23, a part 131 of a power supply plane layer 133 is separated. The separated power supply plane layer 131 is disposed on a substrate 134 that is disposed adjacent to a ground plane layer 132. The power supply plane layer 131 separated from the power supply plane layer 133 is connected with a connecting means 135. Thus, the electrostatic capacity between the separated power supply plane layer 131 and the ground plane layer 132 increases.
In a laminated printed circuit board disclosed as Japanese Patent Laid-Open Publication No. 07-111387, as shown in FIG. 24, a power supply layer 141 and a ground layer 142 are laminated. A slit 143 is diagonally formed on a conductor of at least one of the power supply layer 141 and the ground layer 142.
In a printed circuit board 150 disclosed as Japanese Patent Laid-Open Publication No. 09-205290, as shown in FIG. 25, a power supply layer 151 and a ground layer 152 are disposed on a first surface and a second surface of the printed circuit board 150. At a peripheral portion of the first surface, fine conductor patterns 154 and 155 are alternately formed. At a peripheral portion of the second surface, fine conductor patterns 153 and 156 are alternately formed. The conductor patterns 154 are connected to the ground layer 152. The conductor patterns 155 are connected to the power supply layer 151. The conductor patterns 153 opposite to the conductor patterns 154 are connected to the power supply layer 151. The conductor patterns 156 opposite to the conductor patterns 155 are connected to the ground layer 152.
In a printed circuit board disclosed as Japanese Patent Laid-Open publication No. 09-283974, as shown in FIG. 26, a power supply layer 162 and a first ground layer 163 are oppositely disposed with a dielectric layer 166 and thereby a capacitor C1 is formed. A power supply layer 162 and a second ground layer 164 are oppositely disposed with a dielectric layer 167 and thereby a capacitor C2 is formed. In addition, the first ground layer 163 and the second ground layer 165 are connected through a resistor layer 166.
However, in the cage structure of the printed circuit board shown in FIG. 20, the printed board 101 and the cage 107 function as shields. Thus, when a substrate or a nonconductive cage made of plastic is used, it does not function as a shield. In this case, the cage structure cannot suppress an electromagnetic wave from radiating. In the IC card 111 shown in FIG. 21 and the liquid crystal displaying device shown in FIG. 22, since the printed circuit board requires a metal panel, the mounting density decreases and the fabrication cost increases.
In the printed circuit boards shown in FIGS. 23, 24, 25, and 26, they that are radiation sources suppress undesired electromagnetic waves from radiating. In these printed circuit boards, the voltage fluctuation between the ground for supplying a reference voltage and the power supply layer for supplying a power supply voltage is suppressed. However, in the printed circuit board 130 shown in FIG. 23, even if the power supply plane layer and part of the ground plane layer are adjacently disposed, the increase of the resultant electrostatic capacitance is very small. Thus, the fluctuation of the power supply voltage cannot be sufficiently suppressed. In the printed circuit board 140 shown in FIG. 24, since the slit 143 formed on the power supply layer 141 or/and the ground layer 142 function as slot antennas, the radiation of the electromagnetic waves further increases.
In the printed circuit board 150 shown in FIG. 25, since the polarity of the voltage generated between the conductor patterns 154 and 153 is reverse from the polarity of the voltage generated between the conductor patterns 155 and 156 adjacent to the conductor patterns 154 and 153, the voltages at the end portions of the printed circuit board are offset so as to suppress an electromagnetic wave from radiating. However, since the voltage fluctuation between the power supply layer 151 and the ground layer 152 still takes place, undesired electromagnetic waves radiated from the power supply layer 151 and the ground layer 152 cannot be suppressed. In the laminated printed circuit board 161 shown in FIG. 26, although the radiation of undesired electromagnetic waves and the malfunction of device due to the voltage fluctuation between the power supply and the ground are suppressed, another ground layer 164 and a second dielectric layer 167 should be additionally disposed along with the ground layer 163. Thus, the structure becomes complicated and the cost increases.
In the above-descried related art references, the structures of the printed circuit boards should be largely modified. Thus, to apply the technologies of the above-described related art references to printed circuit boards that are provided as products, they should be redesigned from the beginning.
The present invention is made from the above-described point of view. An object of the present invention is to provide a laminated printed circuit board that suppresses the voltage between a power supply and a ground from fluctuating and effectively suppresses an undesired electromagnetic wave from radiating without need to largely modify a circuit layout of a conventional printed circuit board.